Non-volatile memory device with erase address register

ABSTRACT

A non-volatile memory device includes an array of non-volatile memory cells. The memory has control circuitry to erase the non-volatile memory cells and perform erase verification operations. The memory can be arranged in numerous erasable blocks and/or sub-blocks. An erase register stores data indicating an erase state of corresponding memory sub-blocks. During erase verification, the memory programs the erase register when a non-erased memory cell is located in a corresponding sub-block. Additional erase pulses can be selectively applied to sub-blocks based upon the erase register data. Likewise, erase verification operations can be selectively performed on sub-blocks based upon the erase register data. An address register is provided to store an address of a non-erased memory cell identified during verification. The address from the register is used as a start address for subsequent verification operations on the same array location.

This application is a divisional of U.S. patent application Ser. No.09/802,612 filed Mar. 9, 2001 and titled, “NON-VOLATILE MEMORY DEVICEWITH ERASE ADDRESS REGISTER,” which is commonly assigned andincorporated herein by reference. The present invention relatesgenerally to non-volatile memories and in particular the presentinvention relates to erase operations in a non-volatile memory device.

TECHNICAL FIELD OF THE INVENTION BACKGROUND OF THE INVENTION

Memory devices are typically provided as internal storage areas in thecomputer. There are several different types of memory. One type ofmemory is random access memory (RAM) that is typically used as mainmemory in a computer environment: Most RAM is volatile, which means thatit requires a steady flow of electricity to maintain its contents.Computers often contain a small amount of read-only memory (ROM) thatholds instructions for starting up the computer. An EEPROM (electricallyerasable programmable read-only memory) is a special type non-volatileROM that can be erased by exposing it to an electrical charge. Likeother types of ROM, EEPROM is traditionally not as fast as RAM. EEPROMcomprise a large number of memory cells having electrically isolatedgates (floating gates). Data is stored in the memory cells in the formof charge on the floating gates. Charge is transported to or removedfrom the floating gates by programming and erase operations,respectively.

Yet another type of non-volatile memory is a Flash memory. A Flashmemory is a type of EEPROM that can be erased and reprogrammed in blocksinstead of one byte at a time.

A typical Flash memory comprises a memory array that includes a largenumber of memory cells arranged in a row and column fashion. Each memorycell includes a floating gate field-effect transistor capable of holdinga charge. The cells are usually grouped into erasable blocks. Each ofthe memory cells can be electrically programmed on a random basis bycharging the floating gate. The charge can be removed from the floatinggate by an erase operation. Thus, the data in a cell is determined bythe presence or absence of the charge in the floating gate.

To program a memory cell, a high positive voltage Vg is applied to thecontrol gate of the cell. In addition, a moderate positive voltage isapplied to the drain (Vd) and the source voltage (Vs) and the substratevoltage (Vsub) are at ground level. These conditions result in theinducement of hot electron injection in the channel region near thedrain region of the memory cell. These high-energy electrons travelthrough the thin gate oxide towards the positive voltage present on thecontrol gate and collect on the floating gate. The electrons remain onthe floating gate and function to reduce the effective threshold voltageof the cell as compared to a cell that has not been programmed.

In flash memories, blocks of memory cells are erased as in groups. Thisis achieved by putting a negative voltage on the wordlines of an entireblock and coupling the source connection of the entire block to Vcc(power supply), or higher. This creates a field that removes electronsfrom the floating gates of the memory elements. In an erased state, thememory cells can be activated using a lower control gate voltage.

A common problem with flash memory cells is over-erasure. A cell that iserased past a certain point becomes depleted and cannot be fully turnedoff. That is, too many electrons are removed from the floating gate, andthe memory cell floating gate voltage becomes more positive than thethreshold of the cell. The cell, therefore, cannot be turned off even ifthe control gate is at a ground potential. An over-erased memory cellcan cause all memory cells coupled to the same column to be read aserased cells, even though they may be programmed.

In current flash memory cells, a pre-program cycle is performed on theblock of memory cells prior to performing an erase cycle. As such, allthe cells in a block are first programmed. The cells are then eraseduntil all the cells are completely erased. A threshold voltage (Vt)distribution tightening operation is performed following the eraseoperation to recover memory cells that are over erased. As flash memorydevices increase in memory cell density, the time needed to perform acomplete erase operation also increases.

In flash memories, a substantial part of the erase cycle time is spenton the erase cycle. Out of a typical 1-second erase operation, aboutone-half of the time is spent on pre-programming the memory cells, andthe other half is used on the erase cycle. An erase pulse requires about10 ms, while an erase verification operation requires less than 1 μs.With the density of flash memories increasing, the total time to verifyall the locations is becoming a substantial part of the cycle. Forinstance, in a 64 Megabit flash device organized in 16 erasable blocks,there are four million locations that need to be verified during anerase operation. A typical 1 μs time for each verify cycle results in averify time of 4 seconds. Further, memory cells are being verified forlevels that are much tighter than their regular read levels. Thus, theyneed to be sensed much slower. For instance, a normal read is verifyingthat an erased cell has a threshold level (Vt) that is less than 4.5V.During erase verification, the memory verifies that the cell has a Vtthat is less than 3V. This margin is smaller than prior memories and ismore susceptible to noise.

For the reasons stated above, and for other reasons stated below whichwill become apparent to those skilled in the art upon reading andunderstanding the present specification, there is a need in the art fora flash memory with an improved process for erasing and verifying memorycells.

SUMMARY OF THE INVENTION

The above-mentioned problems with non-volatile memory devices and otherproblems are addressed by the present invention and will be understoodby reading and studying the following specification.

In one embodiment, a non-volatile memory device comprises an array ofnon-volatile memory cells arranged in erasable blocks, and addressregisters associated with the erasable blocks. Each of the addressregisters store a start verification address for its associated erasableblock.

In another embodiment, a non-volatile flash memory device comprises anarray of non-volatile memory cells arranged in erasable blocks. Eacherasable block comprises a plurality of sub-blocks. A plurality ofsub-block start address registers is provided. Each of the plurality ofsub-block start address registers is associated with one of thesub-blocks, and each of the sub-block start address registers isconfigured to store a start address for its associated sub-block. Acontrol circuit verifies an erase state of the non-volatile memory cellsof the array. The control circuitry during operation begins verificationof each sub-block at a memory cell address equal to the sub-block startaddress.

A method of erasing a non-volatile memory device comprises applying anerase pulse to a block of addressable memory cells, and performing anerase verification operation on the block of addressable memory cellsstarting at a memory read address stored in an address pointer circuit.

A method of erasing a flash memory comprises performing a pre-programoperation on a block of memory cells, applying an erase pulse to theblock of memory cells, performing a first erase verification operationon a first sub-block of the block of memory cells, terminating the eraseverification of the first sub-block, and recording an address of amemory cell in the first sub-block when the erase verification of thefirst sub-block is terminated.

A method of verifying a memory array comprises selecting an addressableblock of memory cells, and identifying an address of a memory cell inthe addressable block. The address is greater than a lowest address ofthe selected addressable block. The method includes performing an eraseverification of the memory cell.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of a memory device of an embodiment of thepresent invention;

FIG. 2 illustrates sub-blocks of memory cells and correspondingsub-block registers;

FIG. 3 illustrate an address counter and corresponding address register;

FIG. 4 is a flow chart of an erase operation;

FIG. 5 is a block diagram of a pulse counter and erase registercircuitry;

FIG. 6 is a flow chart of a more detailed erase operation of anembodiment of the present invention; and

FIG. 7 is a detailed flow chart of a verification operation of anembodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

In the following detailed description of the preferred embodiments,reference is made to the accompanying drawings, which form a parthereof, and in which is shown by way of illustration specific preferredembodiments in which the inventions may be practiced. These embodimentsare described in sufficient detail to enable those skilled in the art topractice the invention, and it is to be understood that otherembodiments may be utilized and that logical, mechanical and electricalchanges may be made without departing from the spirit and scope of thepresent invention. The following detailed description is, therefore, notto be taken in a limiting sense, and the scope of the present inventionis defined only by the claims.

Referring to FIG. 1, a block diagram of one embodiment of a synchronousflash memory of the present invention is described. The memory device100 includes an array of non-volatile flash memory cells 102. The arrayis arranged in a plurality of addressable banks. In one embodiment, thememory contains four memory banks 104, 106, 108 and 10. Each memory bankcontains addressable blocks of memory cells. The data stored in thememory can be accessed using externally provided location addressesreceived by address register 112 via address signal connections. Theaddresses are decoded using row decode circuitry 114. Row addresscounter 124 is also provided. The addresses are also decoded using bankdecode logic 116. To access an appropriate column of the memory, columnaddress counter 118 couples addresses to column decode circuitry 122.Data is input and output through I/O circuit 128 via data connections.I/O circuit 128 includes data output registers, an output driver and anoutput buffer. Command execution logic 130 is provided to control thebasic operations of the memory device. A state machine 132 is alsoprovided to control specific operations performed on the memory arrayand cells. The command circuit 130 and/or state machine 132 can begenerally referred to as control circuitry to control read, write, eraseand other memory operations. The data connections are typically used forbi-directional data communication. The memory can be coupled to anexternal processor 200 for operation or testing.

The memory includes a pulse counter circuit to keep track of voltagepulses applied to the memory cells during erase and programmingoperations. The pulse counter is typically used to monitor a totalnumber of pulses applied. In an embodiment of the present invention, thepulse counter is used in conjunction with a pulse register to improveerase operation performance, as explained below.

The memory cell array of a flash memory is typically arranged inerasable blocks. In one embodiment of the present invention, theerasable blocks are further arranged in sub-blocks. For example, a 16Meg memory array can be arranged in four erasable blocks of 4 Meg whereeach of these blocks contain 16 sub-blocks.

During a typical prior art erase operation, an erase pulse, or series oferase pulses, is coupled to an addressed array block. The memoryinternal control, or state machine, then steps through each memory cellof the block to determine if data in the cells are erased (logical 1).The control circuitry stops verifying when it reaches a memory celllocation that is not erased. The memory then applies another erase pulseto the block and begins another verification operation at the firstmemory cell location of the block. This process is repeated until allcells in the erasable block are verified as being in an erased state. Aproblem with this prior art erase/verify operation is that the slowestbit in the block dictates the level of erasure of all the memory cellsin the block. Further, repeated erase pulses can over-erase a memorycell by removing too much charge from its floating gate. In an examplememory, the fastest memory cell requires 5 erase pulses to fully erasethe cell, typical memory cells require 10 pulses to erase, and theslowest memory cell in an erasable block requires 30 pulses to erase.With the prior art erase algorithm, the memory cells that are erasedafter 5 pulses are subjected to an extra 25 erase pulses that couldpotentially make them over-erased.

The present invention performs an erase operation on a block of memorycells uses a forward-looking scan algorithm. After applying erase pulsesto the memory block during a verification operation, the memory beginsforward scanning the memory cells in the block to determine if the cellsare erased. As explained below, the memory does not begin scanning untila predetermined number of erase pulses have been applied to the memoryarray.

Referring to FIG. 2, four sub-blocks 200(a)-(d) of a memory array blockare illustrated. A sub-block register 202 is provided in the memory thathas bits 202(a)-(d), or storage locations, that correspond to eachsub-block. As explained below, the register is used to indicate whichsub-blocks are fully erased and which sub-blocks need additional eraseoperations. The register initially indicates that all of the sub-blocksneed to be erased. When all memory cells of a sub-block have beenverified to be fully erased, the corresponding register bit is changedto protect the sub-block from further erase pulses. The operation of theregister is explained in greater detail below.

The present invention also contains an address verify register that isused to manage the erase verification operation. Referring to FIG. 3, anaddress counter 210 is coupled to a sub-block address verify register220. The sub-block address register can store the contents of theaddress counter for each sub-block. That is, the sub-block addressregister is four-deep and each depth corresponds to one of thesub-blocks. In operation, the address verify register is used to store amemory cell address of the first memory cell in a memory sub-block thatfails an erase verify test. The sub-blocks are sequentially tested todetermine if all memory cells are erased. If a non-erased cell isdetected, its address is stored in the sub-block register. This addressis used in subsequent verification operations as a start address. As aresult, previously verified memory cells are not re-verified.

FIG. 4 is a flow chart illustrating a portion of erase and verifyoperations of an embodiment of the present invention. An erase operationbegins by pre-charging all of the memory cells in a block (260). Thisinsures that all memory cells begin the erase operation withsubstantially the same floating gate charge. The memory bock is thensubjected to one or more erase pulses (262). As explained above, theerase pulses are used to remove charge from the memory cell floatinggates. Care should be taken to avoid over-erasing a memory cell. Thatis, too much charge can be removed from the memory cell such that thememory cell conducts current even when its control gate is turned off.

After the initial erase pulses are applied, a verification of eachsub-block is performed (264). The verification operation includesreading the memory cells of the sub-block to determine that the cellshave been erased. Memory cells are considered erased if they have afloating gate charge (threshold voltage) that is below a predeterminedlevel. When a programmed memory cell is located in a sub-block duringthe verification operation, the memory programs a register bitcorresponding to the sub-block to a first state, such as logic 0 (266).It will be appreciated that the register may be pre-programmed to thefirst state and a subsequent program operation is not required. Afterlocating a cell that is not erased, the memory jumps to the nextsub-block of that erasable block. The memory verifies the next sub-blockand if it finds all the cells in that block are erased, thecorresponding location of the sub-block register is set to a secondstate (such as logic 1) indicating that the sub-block does not requireany further erase pulses. The memory control then continues to verifyall of the sub-blocks of the erasable block.

After all of the sub-blocks in an erase block have been verified, one ormore additional erase pulses are applied to the sub-blocks that have aregister bit indicating that all of the memory cells have not beenerased (268). Additional verify operations are then performed on thesub-blocks that have a register bit programmed to the first state. Thatis, sub-blocks that do not contain programmed memory cells are notsubjected to additional erase pulses. As such, the slowest cell to erasein a block only subjects a small sub-block of memory cells to possibleover-erasure. The erase pulses and verification operations are repeateduntil all of the sub-blocks are fully erased, or a maximum number oferase pulses have been applied.

The threshold voltage (Vt) distribution of the memory cells of thepresent flash memory, relative to prior flash memory cells, is muchtighter. Further, erasing non-volatile memory cells to a very negativethreshold level and then bringing the threshold level back to a higherlevel using recovery methods is not good for memory cell reliability.The present invention provides for a better overall thresholddistribution and improves both the reliability and performance of theerase operation. The above described erase/verify operation can reducethe time needed to perform an erase operation by using registers toidentify a condition of the sub-blocks. The erase operation can befurther improved by tracking a location of the programmed memory cells.

The present invention provides an address pointer for each of thesub-blocks of the memory array. The address pointers are used inconjunction with the memory address counter that is used to stepthroughout the memory array location. During the verification and scanoperations, the memory advances through each memory location and checksfor proper erasure. If the memory finds a location that is not erased,it copies the content of the address counter into the address pointerfor that sub-block. Since there is at least one location in thissub-block that needs further erase pulses, there is no need to continueverifying or scanning that block. As such, the state machine issues acommand to jump to the next sub-block. The memory then jumps to the nextsub-block to continue the proper operation.

The contents of the address pointer for the next sub-block are copied tothe address counter. The address pointer is initially set to a beginningaddress of its corresponding sub-block. The sub-block verification orscan continues until all the cells of the block have been checked by theerase verification operation. On subsequent checks of a sub-block, thememory begins at the location of the first programmed memory cell of theprevious verification check. That is, the address pointer is copied tothe address counter as the start location for each sub-block. When thefull sub-block has been erased, the sub-block register can be set, asexplained above, to indicate that further erase operations are notrequired. By re-starting the address location from the last programmedlocation and jumping to the next sub-block when a programmed cell hasbeen detected within a sub-block, the present invention reduces overheadtime.

As explained above, the flash memory applies an initial erase pulse tothe memory block prior to performing the verification operation. Asexplained above, during the verification operation additional erasepulses are applied to the memory block. It will be appreciated that amemory cell encountered early in the verification operation could resultin numerous erase pulses being applied to the entire block. The presentinvention reduces the risk that a memory cell subjects the memory blockto erases pulses that could result in over-erasure, by usingprogrammable pulse registers. FIG. 5 illustrates state machine 132coupled to a pulse counter 230. A first pulse register 240 and a secondpulse register 250 are coupled to the state machine, and explainedbelow.

The present invention provides non-volatile register 240 that can beprogrammed to instruct the memory to begin scan operations when apredetermined number of erase pulses have been applied to the memoryblock. For example, a statistical evaluation may indicate that X-pulseswill erase a significant number of the memory cells. When thepredetermined number of pulses (X) has been applied to the block duringa verification operation, the memory begins the first scan operation.The register can be programmed based upon a statistical analysis of thememory device during fabrication. Likewise, the register can be modifiedafter fabrication if the memory device characteristics change.

A memory device of the present invention can also include a secondnonvolatile register 250. The second register defines the number oferase pulses that are applied to the memory array sub-blocks thatcontain non-erased memory cells. As explained above, only sub-blocksthat require additional erase pulses are subjected to additional erasepulses during verification operations. The second register is used toinitiate subsequent scan operations when conducting the nextverification operations of the sub-blocks. The contents of the registercan be programmed based upon testing or statistical analysis, asexplained above. The contents of the second register are compared to thepulse counter during verification operations. One skilled in the artwith the benefit of the present disclosure will appreciate thatadditional registers, or processing circuitry, can be used to controlthe number of erase pulses between each verification operation. Forexample, it may be desired to apply up to X-erase pulses during thesecond erase verification operation, while less pulses (such as up toX−1) are desired during a third erase verification operation.

Referring to FIG. 6, a flow chart of an erase and verification operationembodiment is described. The erase operation begins with a pre-chargeoperation (260) to charge all memory cells in a block of the memoryarray. After the pre-charge operation is complete, an erase pulse isapplied to the memory cells in the block (302). A verification operation(306) is then performed on the memory block to determine if the memorycells have been erased. If a programmed memory cell is detected (307),the first pulse register 240 is compared to the pulse counter 230 (304).If the pulse count has not been reached, an additional erase pulse isapplied to the and then the verification operation on the block isresumed by reading each memory cell in each sub-block (306). The scanoperation is detailed below with reference to FIG. 7. If the pulse countis reached, the memory then begins a scan operation (308). The scanoperation is used to read the memory cells in the sub-blocks todetermine if the cells have been erased. If a programmed cell is readduring scanning, additional erase pulses are not applied. The memoryrecords the status of the sub-block and moves to the next sub-block toperform the scan. After the scan operation, the memory applies an erasepulse to the block that are contain a programmed cell (310). The memorythen performs a verification operation on the sub-blocks that containeda programmed cell. Again, the subsequent verification operations beginat the last known programmed cell address. During the verificationoperation additional erase pulses may be needed. The erase pulse counteris compared to the second register (312). The scan operation isperformed when the pulse threshold is reached. The verification and scanoperations continue until the block is fully erased. FIG. 7 illustratesone embodiment of a scan operation 306. During the scan operation of thesub-blocks, the memory determines if the sub-block has its register setto indicate that all cells in the sub-block have been erased (320). Ifthe sub-block is erased, the memory jumps to the next sub-block. Ifthere are sub-blocks remaining to be scanned, the start address for thesub-block is loaded from the address pointer (322). The memory cell atthe start address of the sub-block is read (324). If the memory cell iserased, the cell address is incremented (334) with the address counterif the address is not at the end of the block (336) and the next cell isread (324). If a programmed cell is detected, the cell address is copiedfrom the address counter 220 into the verification address pointer forthe sub-block (328). The memory then jumps to the next sub-block (330).If the memory reaches the end of the sub-block without detecting anyprogrammed cells, the sub-block register 202 is set (338) and the nextsub-block is scanned. The memory completes the scan operation when allsub-blocks have been scanned. Additional erase pulses may be requiredfor all, or some of the sub-blocks, as described above.

CONCLUSION

A non-volatile memory device includes an array of non-volatile memorycells. The memory has control circuitry to erase the non-volatile memorycells and perform erase verification operations. The memory can bearranged in numerous erasable blocks and/or sub-blocks. An eraseregister stores data indicating an erase state of corresponding memorysub-blocks. During erase verification, the memory programs the eraseregister when a non-erased memory cell is located in a correspondingsub-block. Additional erase pulses can be selectively applied tosub-blocks based upon the erase register data. Likewise, eraseverification operations can be selectively performed on sub-blocks basedupon the erase register data. An address register is provided to storean address of a non-erased memory cell identified during verificationand scanning operations. The address from the register is used as astart address for subsequent verification operations on the same arraylocation.

Although specific embodiments have been illustrated and describedherein, it will be appreciated by those of ordinary skill in the artthat any arrangement, which is calculated to achieve the same purpose,may be substituted for the specific embodiment shown. This applicationis intended to cover any adaptations or variations of the presentinvention. Therefore, it is manifestly intended that this invention belimited only by the claims and the equivalents thereof.

What is claimed is:
 1. A method for erasing a flash memory, the methodcomprising: performing a pre-program operation on a block of memorycells; applying a first erase pulse to the block of memory cells;performing a first erase verification operation on a sub-block of theblock of memory cells; terminating the erase verification of thesub-block if a non-erased memory cell is detected; and recording anaddress of the non-erased memory cell when the erase verification of thesub-block is terminated.
 2. The method of claim 1 and furthercomprising: applying a second erase pulse to the sub-block containingthe non-erased memory cell; and performing a second erase verificationoperation on the sub-block starting at the address of the non-erasedmemory cell.
 3. The method of claim 2 wherein the recorded address isloaded into an address register prior to performing the second eraseverification operation on the sub-block.
 4. A flash memory devicecomprising: a flash memory array comprising a plurality of blocks offlash memory cells; and a controller capable of erasing the flash memorydevice by applying an erase pulse to a block of flash memory cells,performing a first erase verification operation on a sub-block of theblock of flash memory cells, terminating the erase verification of thesub-block upon detection of a non-erased memory cell, and recording anaddress of the non-erased memory cell when the erase verification of thesub-block is terminated.
 5. The flash memory device of claim 4 whereinthe controller is a state machine.
 6. An electronic system comprising: aprocessor that generates an erase command; and a flash memory devicecomprising: a flash memory array comprising a plurality of blocks offlash memory cells; and a controller capable of erasing the flash memorydevice in response to the erase command by applying an erase pulse to afirst block of flash memory cells, performing a first erase verificationoperation on a sub-block of the first block of flash memory cells,terminating the erase verification of the sub-block upon detection of anon-erased memory cell, and recording an address of the non-erasedmemory cell when the erase verification of the sub-block is terminated.7. A method for erasing a flash memory, the method comprising: applyingan erase pulse to a block of memory cells; performing a first eraseverification operation on a sub-block of the block of memory cells;terminating the erase verification of the sub-block if a non-erasedmemory cell is detected; and recording an address of the non-erasedmemory cell when the erase verification of the sub-block is terminated.8. A method for erasing a flash memory, the method comprising: applyinga first erase pulse to a block of memory cells; performing a first eraseverification operation on a sub-block of the block of memory cells;terminating the erase verification of the sub-block if a non-erasedmemory cell is detected; recording an address of the non-erased memorycell when the erase verification of the sub-block is terminated;applying a second erase pulse to the sub-block; and performing a seconderase verification starting at the address of the non-erased memorycell.
 9. The method of claim 8 and further including performing apre-program operation on the block of memory cells prior to the firsterase pulse.